Optical second harmonic and sum-frequency generation has evolved into a useful spectroscopic tool for material characterization, especially as a viable and powerful technique for probing surfaces and interfaces. This book serves as an introduction on the technique. It provides a comprehensible description on the basics of the technique and gives detailed accounts with illustrating examples on the wide range of applications of the technique. It clearly points out the unique capabilities of the technique as a spectroscopic tool for studies of bulk and interface structures in different disciplines. This book is an updated version of an earlier book on the same subject, but it puts more emphasis on physical concepts and description. It underscores recent advances of sum-frequency spectroscopy at the technical front as well as over its wide range of applications, with the author's perspective in each area. Most chapters end with a section of summary and prospects that hopefully can help stimulate interest to further develop the technique and explore possibilities of applying the technique.